The present invention is related to a measuring element for a device for determining the mass of a flowing medium, and in particular, the mass of the intake air of internal combustion engines.
German Unexamined Patent Application No. 36 38 137 discusses a device in which resistors and printed conductors are applied to a carrier member, i.e., a substrate. In such devices, also described as air-mass flowmeters, a measuring resistor and compensating resistor having a corresponding reference resistor, together with two balancing resistors, form a Wheatstone bridge. The diagonal bridge voltage of the Wheatstone bridge is applied to an automatic gain-control amplifier. The output voltage from the automatic gain-control amplifier is adapted to trigger a heating resistor element. The heating power is rated to allow the measuring resistor to have a constant temperature. The heating power of the heating resistor is a measure for the mass of the flowing medium.
In the above mentioned device, the substrate is retained on one side in a mounting and divided by a slit into two lips. The slit is oriented at a right angle to the direction of flow and runs, starting from an unattached frontal area of the first and second lip facing away from the mounting, up to near the mounting. The width of the slit corresponds, approximately, to the thickness of the substrate. With its incident-flow surface, the first lip constitutes an obstacle to the flow, and it has a compensating resistor on its rear side. The heating resistor is provided on the front side of the second lip, and the measuring resistor is provided on the rear side of the measuring resistor.
When measuring the mass in the intake area of an internal combustion engine, flow fluctuations exist. These flow fluctuations are caused by the opening and closing of the intake valves of the individual pistons. The intensity of the flow fluctuations, i.e., the pulsation of the flow velocity, is dependent upon the intake frequency of the individual pistons. To a considerable extent, the fluctuations in flow adversely affect the measurement of the device mentioned at the outset, particularly given intense flow fluctuations that entail partial backflow in the measuring duct.
Thus, there exists a need to provide a device for measuring the mass in the intake area of an internal combustion engine which provides a precise measuring result nearly independent of a fluctuating flow.